Prediction And Verification of Wafer Surface Morphology in Ultrasonic Vibration Assisted Wire Saw (UAWS) Slicing Single Crystal Silicon Based On Mixed Material Removal Mode

Monocrystalline silicon is one of the most important semiconductor materials, widely used in chip manufacturing, solar panels. Slicing is the first step in making chips and the surface quality of silicon wafers directly affects the quality of later processing and accounts for a large proportion in the chip manufacturing cost. Ultrasonic vibration assisted wire saw (UAWS) is an effective sawing process for cutting hard and brittle materials such as monocrystalline Si, which can significantly improve the surface quality of silicon wafers. In order to further study the formation mechanism of the surface morphology of single crystal silicon sliced by UAWS, a new model for prediction of wafer surface morphology in UAWS slicing single crystal silicon based on mixed material removal mode is presented and verified in this paper. Firstly, the surface model of diamond wire saw tool is established by equal probability method. Then according to the equation of transverse vibration dynamics about the wire saw with ultrasonic excitation, the trajectory equation of arbitrary abrasive particles on the surface of wire saw is derived and analyzed. Thirdly, a new model for prediction of the wafer surface morphology based on mixed material removal mode is presented, which can be used to predict the wafer surface morphology of single crystal silicon sliced by UAWS. Finally, the prediction model is verified by UAWS slicing experiment, and the effects of slicing parameters such as wire saw speed, feed speed and workpiece rotate speed on the surface quality of silicon wafer were studied. It shows that the predicted wafer surface morphology and the experimental wafer surface morphology are similar in some characteristics, and the average error between the experimental and the theoretical values of the wafer surface roughness is 11.9%, which verifies the validity of the prediction model.

Simulation of the ductile machining mode of silicon

Diamond wire sawing has been developed to reduce the cutting loss when cutting silicon wafers from ingots. The surface of silicon solar cells must be flawless in order to achieve the highest possible efficiency. However, the surface is damaged during sawing. The extent of the damage depends primarily on the material removal mode. Under certain conditions, the generally brittle material can be machined in ductile mode, whereby considerably fewer cracks occur in the surface than with brittle material removal. In the presented paper, a numerical model is developed in order to support the optimisation of the machining process regarding the transition between ductile and brittle material removal. The simulations are performed with an GPU-accelerated in-house developed code using mesh-free methods which easily handle large deformations while classic methods like FEM would require intensive remeshing. The Johnson-Cook flow stress model is implemented and used to evaluate the applicability of a model for ductile material behaviour in the transition zone between ductile and brittle removal mode. The simulation results are compared with results obtained from single grain scratch experiments using a real, non-idealised grain geometry as present in the diamond wire sawing process.

Simulation of the Ductile Machining Mode of Silicon

Diamond wire sawing has been developed to reduce the cutting loss when cutting silicon wafers from ingots. The surface of silicon solar cells must be flawless in order to achieve the highest possible efficiency. However, thesurface is damaged during sawing. The extent of the damage depends primarily on the material removal mode. Undercertain conditions the generally brittle material can be machined in ductile mode, whereby considerably fewer cracksoccur in the surface than with brittle material removal. In the presented paper a numerical model is developed in orderto support the optimization of the machining process regarding the transition between ductile and brittle materialremoval. The simulations are performed with an GPUaccelerated in–house developed code using mesh-free methodswhich easily handle large deformations while classic methods like FEM would require intensive remeshing. Thesimulation results are compared with results obtained from single grain scratch experiments.

Process Parameters Optimization Using Taguchi-Based Grey Relational Analysis in Laser-Assisted Machining of Si3N4

Despite extensive research over the past three decades proving that laser-assisted machining (LAM) is effective for machining ceramic materials, which are affected by many machining parameters, there has been no systematic study of the effects of process parameters on surface quality in LAM ceramic materials. In this paper, the effects and optimization of laser power, spindle speed, feed rate, and cutting depth on surface roughness and work hardening of LAM Si3N4 were systematically studied, using grey relational analysis coupled with the Taguchi method. The results show that the combination of machining parameters determines the material removal mode at the material removal location, and then affects the surface quality. In ductile material removal mode, both the value of surface roughness and work hardening degree are smaller. Decreased surface roughness and work hardening degree can be obtained with smaller cutting depth and higher laser power.

Effects of Depth of Cutting on Damage Interferences during Double Scratching on Single Crystal SiC

In this work, the damage interference during scratching of single crystal silicon carbide (SiC) by two cone-shaped diamond grits was experimentally investigated and numerically analyzed by coupling the finite element method (FEM) and smoothed particle hydrodynamics (SPH), to reveal the interference mechanisms during the micron-scale removal of SiC at variable Z-axis spacing along the depth of cutting (DOC) direction. The simulation results were well verified by the scratching experiments. The damage interference mechanism of SiC during double scratching at micron-scale was found to be closely related to the material removal modes, and can be basically divided into three stages at different DOCs: combined interference of plastic and brittle removal in the case of less than 5 µm, interference of cracks propagation when DOC was increased to 5 µm, and weakened interference stage during the fracture of SiC in the case of greater than 5 µm. Hence, DOC was found to play a determinant role in the damage interference of scratched SiC by influencing the material removal mode. When SiC was removed in a combined brittle-plastic mode, the damage interference occurred mainly along the DOC direction; when SiC was removed in a brittle manner, the interference was mainly along the width of cutting; and more importantly, once the fragment of SiC was initiated, the interference was weakened and the effect on the actual material removal depth also reduces. Results obtained in this work are believed to have essential implications for the optimization of SiC wafer planarization process that is becoming increasingly important for the fabrication of modern electronic devices.

Erosion Properties of Ceramic Composite Material Based on Nano-Mullite Whisker and Zirconia-Toughened Alumina

The improvement of bulk ceramic properties through the addition of a secondary or even tertiary phase is a field of research that has been actively pursued since the mid twentieth century. This pursuit has become more relavent with the adoption of ceramic phases to protect structural components within the hotpath of gas turbines. Improving the properties of these ceramic coatings and tiles has the potential of reducing catastrophic damage events leading to an overall reduction in unplanned maintence and downtime. To date, Several approaches have been undertaken to improve the physical properties of these ceramics including preferential microstructural grain growth and doping to develop metastable crystal phases. This paper examines the effect of whisker additions to a mechanical mixture of oxide ceramics on the erosion properties. The baseline structure is a mechanical mixture of zirconia and alumina particles in the ratio of 89.8vol% alumina to 10.2vol% partially stabalized zirconia. A ratio of 20.0vol% mullite whiskers is incorporated into the structure as a toughening agent. The mullite whiskers are grown using a molten salt method. The overall structural composition is 20.0vol% mullite whiskers, 8.2vol% partially stabilized zirconia, and 71.8vol% alumina. This whisker toughened material is compared to a baseline 89.8vol% alumina 10.2vol% zirconia ceramic. Erosion tests were conducted using a 50 μm diameter alumina erodant with a velocity of 104 m/s. Impingement angles of 30°, 60° and 90° were examined to determine the effect of whisker additions at steeper attack angles. Despite the increased hardness, tensile strength and fracture toughness, whisker-enhanced zirconia-toughened alumina has shown similar erosion rate as non-reinforced ZTA at 30° ad 90° and much higher erostion at 60°. It can be surmised that whisker-enhanced toughening of ceramics has little positive effect on the erosion resistance of ceramics at room temperature and is potentially harmful. Microstructure analysis results are also presented within to illustrate the erosion material removal mode under different conditions.